DIAGNOSIS The warmer plate thermostat or its control relay is stuck in the "on" position. This is a failure of the warming circuit.
FIX Unplug the machine. The warmer plate thermostat is located under the warming plate. Access requires disassembly. Test the thermostat for continuity when cold; it should be closed. If it reads open when cold, it is faulty and must be replaced with an identical part.
IF THAT DIDN'T WORK The issue could be the triac or relay on the control board that switches power to the warmer plate. A failed triac will send constant power. The control board may need replacement.
HEADS UP A stuck-on warmer plate is a fire hazard and can crack the glass carafe. Unplug the machine immediately and do not use it until repaired.